The present invention relates to a moving distance detector in an elevator and, more particularly, to improvements in an encoder pulse receiver in a device for detecting the moving distance of an elevator cage by an encoder.
A counter for counting a pulse signal output from a rotary encoder has been used in general for digitally detecting the moving distance of an elevator cage. FIG. 3 shows an example of a method of measuring the moving distance of an elevator cage using the counter. In FIG. 3, reference numeral 1 denotes an electric motor, numeral 2 denotes a drive sheave of a winch driven by the motor 1, numeral 3 denotes a main cable engaged with the sheave 2, and a cage 4 and a balance weight 5 are engaged with both ends of the main cable 3.
Reference numeral 6 denotes a rotary encoder (hereinbelow referred to as "an encoder") for outputting a pulse signal in response to the rotation of the motor 1. This pulse signal is input through a transmission line 6A to an encoder pulse receiver 7, and then fed to a counter 8 which counts the signal pulses to detect the moving distance of the cage 4 on the basis of the counted value.
The internal configuration of the encoder 6 and the encoder pulse receiver 7 described above are shown in FIG. 4. In FIG. 4, the encoder 6 comprises a disk 61 formed with a plurality of light transmitting slits radially formed along the circumferential direction thereof, so as to rotate as the motor 1 rotates, a photoelectric device 62 for detecting the light passed through the slits to transmit a pulse detection signal responsive to the rotation of the motor 1, and a differential driver 63 for transmitting the detection signals as differential pulse signals V.sub.A, V.sub.B.
A differential amplifier 71 forms an encoder pulse receiver 7, receives the differential pulse signal and outputs a pulse signal V.sub.C responsive to the pulse detection signal, and terminating resistors 72 and 73 for biasing input terminals A.sub.1 and B.sub.1 to H and L levels, respectively, when no signal is input are respectively provided between the first input terminal A.sub.1 of the differential amplifier 71 and a power source V, and between the second input terminal B.sub.1 and ground (or a negative power source). Reference character 6A denotes a signal transmission line formed of two wires of signal transmission lines 6a and 6b.
The operation of the conventional moving distance detector of an elevator constructed as described above will be described with reference to FIGS. 1 to 4 together with FIG. 5. FIG. 5 is a waveform diagram of pulse signals presented at the input terminals A.sub.1, B.sub.1 and the output terminal C.sub.1 of the encoder pulse receiver.
A pulse detection signal output from the photoelectric device 62 in response to the rotation of the motor 1 is converted by the differential driver 63 to the differential pulse signals V.sub.A, V.sub.B having relative logic levels (H and L levels), and then input from output terminals A.sub.0, B.sub.0 through the signal transmission lines 6a and 6b to the input terminals A.sub.1 and B.sub.1 of the differential amplifier 71 of the encoder pulse receiver 7.
Since the logic levels (H or L) of the differential pulse signals V.sub.A and V.sub.B input from the input terminals A.sub.1 and B.sub.1 are opposite, the differential receiver 71 amplifier the signals as differential signals and outputs a pulse signal V.sub.C responsive to the difference. As a result, even if a noise is induced from the signal transmission line 6A to the differential pulse signals V.sub.A and V.sub.B while the differential pulse signals V.sub.A, V.sub.B are transmitting from the encoder side through the signal transmission line 6A, the noise is removed from the differential signals V.sub.A, V.sub.B because the noise is input to the differential amplifier 71 together with the differential pulse signals V.sub.A, V.sub.B as the same phase components, and the pulse signal V.sub.C of the output is not affected by the noise. Further, when one of the differential pulse signals V.sub.A, V.sub.B is not transmitted due to the disconnection of one of the signal transmission lines 6a or 6b, the input terminals A.sub.1 or B.sub.1 of the differential amplifier 71 connected to the disconnected signal line is biased by the terminating resistor 72 or 73 to the H or L level potential of the differential pulse signal. The differential amplifier 71 inputs the biased potential and one differential pulse signal transmitted through the normal signal line, and outputs a pulse signal V.sub.C. The pulse signal V.sub.C is counted by the counter 8 which detects the moving distance of the cage 4 on the basis of the counted value.
Since the conventional moving distance detector of the elevator using a balancing transmission system is constructed as described above, when one signal is not transmitted due to the disconnection of the signal transmission lines 6a, 6.sub.b or improper connection of the connector connecting the signal transmission lines 6a, 6b to the encoder 6 or the encoder pulse receiver 7, such as, for example, when the signal transmission line 6b is disconnected, the input terminal B.sub.1 of the differential amplifier 71 is biased by the terminating resistor 73 substantially to L level V.sub.L, and when the signal level of the input terminal A.sub.1 becomes the L level V.sub.L, the signals of the both input terminals substantially coincide. However, since the signal level of the output terminal C.sub.1 is determined by only a slight difference of the signal level between the input terminals A.sub.1 and B.sub.1, the pulse signal output from the differential amplifier 71 is erroneously generated and becomes very unstable. As a result, more than a required predetermined number of pulses are generated, or less pulses are generated. Consequently, there arises various problems that the value of the pulses counted by the counter 8 does not coincide with the moving distance of the cage 4, the elevator cannot be correctly controlled, and that the problem cannot be readily discovered.